The vacuum gauges used to this effect are known to be ionization gauges which measure the pressure of the ultra-vacuum from the density of the particles the ultra-vacuum atmosphere contains, said atmosphere being itself contained in a specific recipient. The known principle for functioning of these gauges consists of ionizing one proportion of the molecules and atoms of the gas constituting this atmosphere via impact with electrons derived from a source whose flowrate is known. The gas ions formed are then collected by a measuring electrode of the system and the ionic current obtained is an analogous measurement of the pressure of the ultra-vacuum existing in the chamber.
It is also known that the formation of said molecules takes place in two different ways. In ionization vacuum gauges, known as cold cathode or Penning gauges, the ions are formed in a maintained electric discharge and at high intensity in the presence of a homogeneous magnetic field. In another category of vacuum gauges, ionization is produced via the collision with electrons emitted by a heated filament, these ionization vacuum gauges being hot cathode ionization vacuum gauges. The present invention concerns a vacuum gauge derived from a type of hot cathode vacuum gauges known under the name of Bayard-Alpert vacuum gauges, reference firstly being made to the principle associated with FIG. 1 which describes a device of the prior art.
The Bayard-Alpert gauge described in FIG. 1, which is an exploded skeleton diagram, mainly includes a chamber 1 containing the ultra-vacuum atmosphere whose pressure is desired to be determined. This chamber 1 contains the three electrodes of the system, namely a hot cathode or filament 2 intended for the emission of a flow of electrons, an anode grid 3 and, along the axis of the chamber 1, an ions collector 4. In the example shown, the anode grid surrounds the collector which is disposed along the axis of the chamber 1, but this is not strictly the case for all Bayard-Alpert type gauges. This gauge operates at low voltage and in the absence of any magnetic field. The thermoelectronic electrons emitted by the cathode 2 are accelerated in the electric field created by the polarization of the anode 3 and acquire there sufficient energy so as to ionize the gas contained in the inter-electrode system. The positive ions created by impacts are attracted by the collector 4 found at a potential close to that of the chamber and the current thus produced makes it possible to measure the pressure. The electrons are finally captured by the anode 3 which most frequently appears in the form of a helical wire. The hot cathode is a highly productive source of electrons which enables high sensitivity to be obtained, particularly at extremely low pressure. Moreover, hot cathode gauges have, as compared with cold cathode gauges, a much weaker pumping effect by an order of magnitude.
The upper pressure limit able to be measured by this system is several 10.sup.-3 mbars; this is mainly due to the fact that, at higher pressures, electric arcs or luminous discharges may occur. The filament then may possibly burn. At lower pressures, the measurement is limited by two physical effects, namely:
the desorption effect of the anode under the influence of the electrons striking it. This effect is independent of the pressure but is proportional to the electronic emission current. PA1 the Rontgen effect: when the electrons strike the anode, they free photons (soft X-radiation) which in turn create photoelectrons when striking various surfaces, including the collector. The photoelectrons freed by the collector flow to the anode, thus creating a current of the same direction as the ionic current. For Bayard-Alpert gauges, this effect becomes preponderant within the range of 10.sup.-10 mbars. PA1 one first d.c. power unit to positively polarize at Vc the cathodic electrodes of the electron source with respect to the chamber, PA1 one second d.c. power unit for polarizing at Vg the electron extraction grids to a variable positive potential with respect to the cathodic electrodes, PA1 one third d.c. power unit for positively polarizing at Va the electron collecting anode grid with respect to the electron extraction grid, said three power units being connected in series between the chamber and the electron collection anode grid, PA1 one first current measuring device mounted between the second and third power units for measuring the electronic current, PA1 a second current measuring device mounted between the ions collector and the chamber so as to measure the ionic current, PA1 processing means connected to the first and second current measuring devices so as to calculate the pressure of the ultra-vacuum existing in the chamber on the basis of the read values of the electronic current and the ionic current.
The hot cathode 2 is formed of a filament located outside the volume delimited by the grid. The electrons emitted by the cathode go backwards and forwards through the grid until they are trapped. By way of example, for a vacuum of 10.sup.-8 mbars and an electronic current of 1 mA, an ionic current of 10 pA is obtained (for a gauge factor of 10 mbars.sup.-1).
The main defect of hot cathode gauges derives from the systematic use of a heating filament as a source of electrons.
This type of thermoelectronic source is in fact isotropic, whereas the directivity of the beam of electrons is an important parameter as regards gauge sensitivity. In fact, it has been shown that the average length of the electron trajectories is that much longer when the latter are radial, that is directed towards the collector 4, and it is known that the probability of ionization of the gas inside the zone delimited by the grid 3 is directly proportional to this average length (the ionization probability increases with the length). It is solely to the extent that the field is fully homogeneous around the filament 2 that the beam is reconcentrated by the grid; this is why the sensitivity of the guage is highly dependent on the position of the filament 2, which in addition may move during the time period or sag under the effect of the heat (about 2000.degree. C.), all the more so when the electrons emitted by the filament leave the latter with a kinetic energy of almost nil and without directivity. Similarly, the fact that the source is relatively extended and that all its points do not emit in a similar way renders it more difficult to reconcentrate the beam of electrons and adversely affects its regularity. These two reasons mean that the sensitivity of the gauge is time-unstable parameter and to a certain extent cannot be reproduced. In addition, the electron emission phenomenon is originally thermic and costly in energy terms, has an extremely long response time and, in certain applications, is a significant pollutive factor.
Another concept for measuring the vacuum is described in the British review "Discovery", vol. 25, No 10, October 1964, p.15-16.
This structure, able to measure vacuum absolutes, uses the emission of electrons by tungsten points, with a diameter of several thousands of Angstrom units, used as emissive cathodes and comprising the cut extremity of macroscopic metallic rods, similar to nails. These macropoints and a polarized anode are placed in the chamber where it is desired to measure the vacuum degree. An electric current is thus established between the points constituting the cathode and the polarized anode. The recommended method consists of measuring the fluctuations of this current due to the gas atoms which are fixed to the points and are regarded as representative of the number of atoms present in the chamber, that is ultimately of the pressure. Apart from the fact that, at extremely low pressure, a large amount of time (several hours) is required to be able to correctly measure the pressure, such a system is difficult to use owing to the high instability of the cathodic source established.